(1) Field of the Invention
The invention deals with an apparatus for and a method of determining the mechanical properties of sheet materials under conditions of edgewise compressive loading.
(2) Description of the Prior Art
Thin sheets of materials such as paper or cardboard are frequently used in packaging or other applications where their mechanical properties--such as modulus of elasticity, ultimate strength, strain to failure, creep rate and dimensional stability--are important. In attempting to test a thin sheet specimen in edgewise compression, unless adequate lateral restraint is provided, the sheet shifts out of plane or "buckles" at such a light load that it is impossible to determine the engineering properties of the material. The testing and buckling of such sheets can be understood simply by grasping an ordinary sheet of paper, placing one hand on one width of the paper and the other hand on the other width. Moving one's hands towards each other is analagous to applying an edgewise compressive loading to this sheet, and the results are the same in each case. The sheet will shift laterally, or out of plane.
In order that measurements of such sheet materials' properties can be made, various methods are currently used. One alternative is to roll the sheets into tubes or cylinders and provide them with internal support. Secondly, the span or length of the specimen is shortened to the point where buckling does not occur. Thirdly, the sheet is restrained between two flat plates. Finally, the sheet is restrained between an array of fingers on each side of the specimen sheet. These fingers deflect with the sample as it undergoes strain in the direction of loading, but provide restraint against lateral motion. These methods, however, have several significant shortcomings.
When a sheet must be rolled to form a cylinder before testing, undesirable transverse stresses occur. When a specimen is unduly short, measurement of its properties cannot be made on its body but must instead be made on the clamps holding the specimen in place.
Errors in measurement are then possible due to clamping effects on the paper, restraint of deformation in the transverse direction, and slipping of the specimen in the clamps. When a specimen is tested between two flat plates, frictional forces must occur but cannot be accurately measured. Finally, measurement of the properties of a specimen held on both sides by fingers can also not be made on the specimen body, resulting in errors of the same kind as those occuring in measurement of a very short specimen's properties.